High-efficiency energy harvesting interface and corresponding energy harvesting system

ABSTRACT

An electrical-energy harvesting system envisages a transducer for converting energy from an environmental energy source into a transduced signal, an electrical energy harvesting interface for receiving the transduced signal and for supplying a harvesting signal, and an energy storage element coupled to the electrical energy harvesting interface for receiving the harvesting signal. The electrical-energy harvesting system also includes a voltage converter connected to the electrical energy harvesting interface for generating a regulated voltage. The harvesting interface samples an open-circuit voltage value of the transduced signal, generates an optimized voltage value starting from the open-circuit voltage value, and generates an upper threshold voltage and a lower threshold voltage on the basis of the optimized voltage value. The harvesting interface controls the voltage converter in switching mode so that the harvesting signal has a value between the upper and lower threshold voltages in at least one operating condition.

BACKGROUND

1. Technical Field

The present disclosure relates to a high-efficiency energy harvesting interface and to a corresponding energy harvesting system.

2. Description of the Related Art

As is known, systems for harvesting (or scavenging) energy from mechanical or environmental energy sources arouse considerable interest in a wide range of technological fields, for example in the field of portable electronic devices or in the automotive field.

Typically, energy harvesting systems are designed to harvest, store, and transfer energy generated by mechanical or environmental sources to a generic electrical load, which may be supplied, or, in the case of a battery, recharged. These systems thus enable production of electronic apparatuses that operate without a battery, or with a considerable increase in the duration of batteries in the case of apparatuses provided therewith.

For harvesting environmental energy, solar or thermoelectric generators may be used, which convert solar energy and thermal energy, respectively, into electrical energy.

FIG. 1 shows schematically and by functional blocks, an energy harvesting system of a known type.

The energy harvesting system 1 comprises a transducer 2, for example a photovoltaic or thermoelectric generator that includes a plurality of cells (of a known type, not described in detail herein), which converts solar energy or thermal energy into electrical energy, typically into a DC voltage or, in any case, into a voltage that varies slowly in time (with respect to the electrical constants of the circuit), generating a transduction signal V_(TRANSD).

The energy harvesting system 1 further comprises a harvesting interface 4, designed to provide a condition of coupling with the transducer 2 of the MPPT (Maximum Power Point Tracking) type, in order to maximize extraction of power. The harvesting interface 4 is configured to receive at input the transduction signal V_(TRANSD) generated by the transducer 2 and supply at output a harvesting signal VIN_(DCDC).

The energy harvesting system 1 further comprises: a storage capacitor 5, which is connected to the output of the harvesting interface 4 and receives the harvesting signal VIN_(DCDC), which determines charging thereof and consequent storage of energy; and a DC-DC converter 6, connected to the storage capacitor 5 for receiving at input the stored electrical energy and generating at output a regulated signal V_(REG), with an appropriate value so that it may be supplied to an electrical load 8, for its supply or its recharging.

The global efficiency η_(TOT) of the energy harvesting system 1 is given by the expression:

η_(TOT)=η_(TRANSD)·η_(MPPT)·η_(DCDC)

where: η_(TRANSD) is the efficiency of the transducer 2, indicating the amount of environmental energy, effectively converted by the transducer 2 into electrical energy; η_(MPPT) is the efficiency of the harvesting interface 4, indicating the amount of converted electrical energy that is effectively used for charging the storage capacitor 5; and η_(DCDC) is the efficiency of the DC-DC converter 6.

In particular, the efficiency η_(MPPT) of the harvesting interface 4 indicates the ratio between the power effectively transferred onto the storage capacitor 5 and the maximum power that could theoretically be supplied, P_(MAX).

In detail, this efficiency η_(MPPT) is given by the following expression:

η_(MPPT)=η_(COUPLE)·η_(LOSS)

where η_(COUPLE) is the coupling factor between the transducer 2 and the harvesting interface 4 (indicating the impedance matching between the same transducer 2 and the harvesting interface 4), and η_(LOSS) is the loss of power due to consumption by the harvesting interface 4.

It has been shown that, in the case of a thermoelectric cell, which may be represented schematically, as illustrated in FIG. 2 a, as an equivalent voltage generator V_(OC) connected to a series resistance R_(TEG), the efficiency η_(MPPT) is maximized in the case where the transduction signal V_(TRANSD) has an optimized value V_(MPPT) equal to V_(OC)/2 (i.e., a value equal to one half of the load-less, or open-circuit, voltage supplied by the corresponding equivalent voltage generator).

Likewise, in the case of a photovoltaic cell, which may be represented schematically, as illustrated in FIG. 2 b, as an equivalent current generator I_(PV) connected in parallel to a diode D_(PV) (in the figure, the equivalent series resistance of the generator is not represented), the efficiency η_(MPPT) is maximized in the case where the transduction signal V_(TRANSD) has an optimized value V_(MPPT) comprised between 0.75·V_(OC) and 0.9·V_(OC) (according to the constructional parameters of the photovoltaic cell and the material of which it is made), for example equal to 0.8·V_(OC), where V_(OC) is once again the open-circuit voltage supplied by the photovoltaic cell.

It is consequently required that the harvesting interface 4 of the energy harvesting system 1 be configured in such a way that the transducer 2 operates in, or around, a working point that ensures the aforesaid condition of maximum efficiency.

For this purpose, a wide range of circuit configurations have been proposed for providing the harvesting interface 4.

For instance, in the document entitled “A Seamless Mode Transfer Maximum Power Point Tracking Controller for Thermoelectric Generator Applications” by Rae-Young Kim, Jih-Sheng Lai, IEEE Transactions on Power Electronics, vol. 23, No. 5, September 2008, an interface circuit has been proposed, comprising a dual voltage conversion stage, formed by the cascade of a boost converter and a buck converter, the latter being designed to regulate the value of the output voltage. Tracking of the MPPT condition is obtained with a continuous-time control of the duty cycle of the boost converter.

The present Applicant has, however, realized that this solution involves a high power consumption, which is due to the fact that the control is of a continuous-time type, which does not render it suited to energy harvesting applications. Further, this solution does not prove flexible, being suited only to a specific type of transducer and to precise values of the electrical parameters associated thereto, further depending upon the tolerance in the values assumed by the same electrical parameters. In general, this solution also involves a large number of external components, which may not be made with integrated technology.

Another possible circuit implementation is described in the document entitled “Thermoelectric Energy Harvesting with 1 mV Low Input Voltage and 390 nA Quiescent Current for 99.6% Maximum Power Point Tracking” by Chao-Jen Huang, Wei-Chung Chen, Chia-Lung Ni, Ke-Horng Chen, Chien-Chun Lu, Yuan-Hua Chu, and Ming-Ching Kuo, 38th European Solid-State Circuits Conference (ESSCIRC), September 2012. This solution envisages a boost converter and a continuous-time algorithm, the so-called perturbation and observation algorithm, to achieve the MPPT condition; in particular, the duty cycle of the converter is perturbed, and the trend of the output voltage is measured: the MPPT condition corresponds to a maximum positive trend.

The present Applicant has, however, realized that also this solution has some disadvantages, amongst which: a high power consumption, intrinsic in a continuous-time perturbation and observation algorithm, which renders it difficult to use in energy harvesting applications; and a poor efficiency, when combined to a low-power transducer.

The document entitled “A Coreless Maximum Power Point Tracking Circuit of Thermoelectric Generators for Battery Charging Systems”, by S. Cho, N. Kim, S. Park, S. Kim, IEEE Asian Solid-State Circuits Conference, Nov. 8-10, 2010, Beijing, China, describes yet a further solution for providing the harvesting interface. This solution envisages two conversion stages, with the cascade of a boost conversion stage and a buck conversion stage, the latter for regulation of the output voltage; the MPPT condition is achieved by a control of the switch in the boost stage.

The present Applicant has realized that also this solution, albeit presenting a simpler algorithm to achieve the MPPT condition, does not have a high efficiency, on account of the presence of two conversion stages. Further, also this solution requires a large number of external components, not made with integrated technology.

The subject matter discussed in the Background section is not necessarily prior art and should not be assumed to be prior art merely as a result of its discussion in the Background section. Along these lines, any recognition of problems in the prior art discussed in Background section or associated with such subject matter should not be treated as prior art unless expressly stated to be prior art. Instead, the discussion of any subject matter in the Background section should be treated as part of the inventor's approach to the particular problem, which in and of itself may also be inventive.

BRIEF SUMMARY

The present disclosure provides an energy harvesting interface that will enable the aforementioned problems and disadvantages to be overcome, in full or in part, and in particular that will provide a high efficiency.

According to the present disclosure, an energy harvesting system provided with an energy harvesting interface is consequently provided, as defined in the annexed claims.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments are described with reference to the following drawings, wherein like labels refer to like parts throughout the various views unless otherwise specified. For a better understanding of the present disclosure, preferred embodiments thereof are now described purely by way of non-limiting example and with reference to the attached drawings, wherein:

FIG. 1 shows an energy harvesting system according to a known embodiment;

FIGS. 2 a and 2 b show the equivalent circuit diagrams of a transducer, including a thermoelectric cell or a photovoltaic cell, respectively;

FIG. 3 shows a circuit diagram of an energy harvesting system according to one embodiment of the present solution;

FIG. 4 shows a flowchart relating to operations performed by a harvesting interface in the system of FIG. 3;

FIGS. 5 and 6 a-6 b show plots of electrical quantities associated with the harvesting interface;

FIG. 7 shows a more detailed circuit diagram of one embodiment of the harvesting interface;

FIGS. 8 a-8 d show plots relating to the electrical performance of the harvesting interface; and

FIG. 9 shows, by way of example, an electronic device, in particular a bracelet or an electronic watch, including the energy harvesting system.

DETAILED DESCRIPTION

As illustrated in FIG. 3, an energy harvesting system 10 according to one embodiment of the present solution, comprises in general, and substantially as has been described previously: a transducer 12, in particular a photovoltaic or thermoelectric generator, which generates a transduction signal V_(TRANSD), in particular a DC voltage or a slowly varying voltage; a harvesting interface 14, which provides a MPPT coupling, receives the transduction signal V_(TRANSD), on an input terminal 14 a, and supplies a harvesting signal VIN_(DCDC), on an output terminal 14 b; a storage capacitor 15, which is connected to the output terminal 14 b of the harvesting interface 14 and receives the harvesting signal VIN_(DCDC); and a DC-DC converter 16, connected to the storage capacitor 15 for receiving the electrical energy stored and generating at output a regulated signal V_(REG), which is then supplied to an electrical load (here not illustrated), for its supply or recharge.

According to one aspect of the present solution, and as will also be described in detail hereinafter, the harvesting interface 14 comprises: a tracking switch SW_(MPPT), which is connected between the input terminal 14 a and the output terminal 14 b of the harvesting interface 14 and is controlled by a control signal V_(SWMPPT); a sample-and-hold (S&H) stage 22, configured to sample the value V_(OC) of the transduction signal V_(TRANSD), generated by the transducer 12 in an open-circuit or loadless condition, at appropriate time intervals, to generate the optimized value V_(MPPT) (see the foregoing discussion) starting from the value V_(OC), and to generate an upper threshold voltage VTH_(UP) and a lower threshold voltage VTH_(DOWN), which satisfy the relation VTH_(DOWN)<V_(MPPT)<VTH_(UP), on the basis of the optimized value V_(MPPT); a comparator stage 24, with hysteretic voltage control, on the basis of the upper and lower threshold voltages VTH_(UP), VTH_(DOWN), which generates at output an enabling signal EN_(DCDC) for the DC-DC converter 16; and a timing stage 25, which generates appropriate control and timing signals for operation of the harvesting interface 14, amongst which the aforesaid control signal V_(SWMPPT).

In general, operation of the harvesting interface 14 envisages, upon opening of the tracking switch SW_(MPPT), sampling of the value of the transduction signal V_(TRANSD), with the transducer 12 operating in loadless conditions, and corresponding generation of the upper and lower threshold voltages VTH_(UP), VTH_(DOWN); and subsequently, upon closing of the tracking switch SW_(MPPT), generation of the harvesting signal VIN_(DCDC), with a value comprised between the upper and lower threshold voltages VTH_(UP), VTH_(DOWN) (thanks to the hysteretic control by the comparator stage 24), and thus about the optimized value V_(MPPT). Given that, for closing of the tracking switch SW_(MPPT), the value of the harvesting signal VIN_(DCDC) coincides with the value of the transduction signal V_(TRANSD), the condition of energy transfer into the storage capacitor 15 thus occurs in an MPPT condition, with substantially maximum efficiency and substantially maximum coupling between the transducer 12 and the harvesting interface 14.

Conveniently, the S&H stage 22 is controlled by the timing stage 25 for sampling and periodically refreshing the value V_(OC) and consequently the values of the upper and lower threshold voltages VTH_(UP), VTH_(DOWN) in such a way as to react promptly and adapt to possible variations of the operating conditions of the transducer 12.

In detail, the S&H stage 22 of the harvesting interface 14 comprises: a sampling switch SW_(S&H), which is connected between the input terminal 14 a of the harvesting interface 14 and a first internal node N1 and receives a control signal V_(SWS&H); a voltage divider 30, formed by a first dividing resistor R1 _(S&H), connected between the first internal node N1 and a second internal node N2, and a second dividing resistor R2 _(S&H), connected between the second internal node N2 and a reference terminal, or ground, GND (both dividing resistors R1 _(S&H), R2 _(S&H) have a resistance value much higher than the value of the series resistance of the equivalent generator of the transducer 12); a first decoupling switch SW1, which is connected between the second internal node N2 and a third internal node N3 and receives a control signal V_(SW1); a first holding capacitor C1 _(S&H), connected between the third internal node N3 and the reference terminal GND; a first voltage-generator module 32, which is connected between the third internal node N3 and a fourth internal node N4 and is designed to generate an offset voltage V_(OS); a second voltage-generator module 34, which is connected between the third internal node N3 and a fifth internal node N5 and is designed to generate the same offset voltage V_(OS); a second holding capacitor C2 _(S&H), connected between the fourth internal node N4 and the reference terminal GND; and a third holding capacitor C3 _(S&H), connected between the fifth internal node N5 and the reference terminal GND.

The comparator stage 24 comprises: a comparator 35, including in a per se known manner an appropriately configured operational amplifier, having a first input terminal connected to the output terminal 14 b of the harvesting interface 14, a second input terminal, and an output terminal, which is connected to an enabling input of the DC-DC converter 16 and is designed to supply the enabling signal EN_(DCDC); a first comparison switch SW1 _(COMP),l which is connected between the fourth internal node N4 of the harvesting interface 14 and the second input terminal of the comparator 35 and receives a control signal V_(SW1COMP); and a second comparison switch SW2 _(COMP), which is connected between the fifth internal node N5 of the harvesting interface 14 and the second input terminal of the comparator 35, and receives, as a control signal, the enabling signal EN_(DCDC).

The timing stage 25, including in a per se known manner (not described in detail herein), an oscillator circuit, is configured to generate the control signals V_(SWMPPT), V_(SWMS&H), V_(SW1), V_(SW1COMP) for the switches SW_(MPPT), SW_(MS&H), SW1, SW1 _(COMP), according to a timing algorithm described in detail hereinafter. As will be discussed hereinafter, the timing stage 25 may supply further control signals for further switches that may be present in the circuit.

With reference also to the flowchart of FIG. 4, operation of the harvesting interface 14 envisages that in an initial step 40 the value of the loadless or open-circuit voltage V_(OC) of the transduction signal V_(TRANSD) supplied by the transducer 12 is sampled and held.

For this purpose, the tracking switch SW_(MPPT) is driven into the opening condition, and the sampling switch SW_(S&H) is driven into the closing condition; in this step, the DC-DC converter 16 is turned off, and the first decoupling switch SW1 is further driven into the closing condition.

The transducer 12 operates substantially in an open-circuit condition, given that the resistance as a whole supplied by the voltage divider 30 is much higher than its own equivalent series resistance, so that the value of the transduction signal V_(TRANSD) that is supplied and that is present on the first internal node N1 substantially coincides with the loadless or open-circuit voltage V_(OC).

In this situation, the voltage divider 30, by an appropriate choice of the division ratio, generates on the second internal node N2 a sampled voltage V_(S&H) having a value substantially equal to V_(OC)/2, in the case where the transducer 12 implements a thermoelectric cell, or comprised between 0.75·V_(OC) and 0.9·V_(OC), for example substantially equal to 0.8·V_(OC), in the case where the transducer 12 implements, instead, a photovoltaic cell.

In any case, the value of the sampled voltage V_(S&H) corresponds to the value that the transduction signal V_(TRANSD) supplied by the transducer 12 assumes in a maximum efficiency or maximum coupling operating condition, i.e., to the optimized value V_(MPPT), thus depending upon the electrical and constructional characteristics of the same transducer 12.

The first holding capacitor C1 _(S&H) is consequently charged to the aforesaid optimized value V_(MPPT) assumed by the sampled voltage V_(S&H).

In detail, the sampled voltage V_(S&H) is given by the following expression:

V _(S&H) =V _(OC) ·R2_(S&H)/(R1_(S&H) +R2_(S&H))

Consequently, the values of resistance of the dividing resistors R1 _(S&H), R2 _(S&H) are set, or regulated, in such a way that:

R2_(S&H) =R1_(S&H) ; R2_(S&H)/(R1_(S&H) +R2_(S&H))=½

in the case where the transducer 12 implements a thermoelectric cell, and for example:

R2_(S&H)=4·R1_(S&H) ; R2_(S&H)/(R1_(S&H) +R2_(S&H))=0.8

in the case where the transducer 12 implements a photovoltaic cell, having a maximum efficiency in the condition V_(MPPT)=0.8·V_(OC).

It is noted that it is thus advantageous to provide at least one, or both, of the dividing resistors R1 _(S&H), R2 _(S&H) so that their resistance is configurable for generating the optimal value for the voltage V_(MPPT).

Next (step 42), once again with the tracking switch SW_(MPPT) open and the DC-DC converter 16 turned off, the values of the upper and lower threshold voltages VTH_(UP), VTH_(DOWN), at which the second and third holding capacitors C2 _(S&H), C3 _(S&H),l respectively, are charged, are generated

VTH_(UP) =V _(S&H) +V _(OS); and

VTH_(DOWN) =V _(S&H) −V _(OS).

Then (step 44), the sampling switch SW_(S&H) is driven into the opening condition, as likewise the first decoupling switch SW1. In this way, the voltage values stored in the holding capacitors C2 _(S&H), C3 _(S&H) are held, but for the leakage currents, which are in any case minimized with an appropriate design of the switching elements in the circuit.

Furthermore, the tracking switch SW_(MPPT) is driven into the closing condition, thus starting the step of tracking of the value of the transduction signal V_(TRANSD), which enables substantially maximum efficiency and substantially maximum coupling to be obtained. The first comparison switch SW1 _(COMP) is further driven into the closing condition so that the second input terminal of the comparator 35 is at the upper threshold voltage VTH_(UP).

As mentioned previously, the tracking step envisages that the DC-DC converter 16 is turned on/turned off via the hysteretic control of the comparator 24, which generates the enabling signal EN_(DCDC), causing the harvesting signal VIN_(DCDC) (and consequently the transduction signal V_(TRANSD), given the presence of the short circuit defined by the tracking switch SW_(MPPT) in a closing condition) to have a variable trend between the upper and lower threshold voltages VTH_(UP), VTH_(DOWN), thus around the sampled voltage V_(S&H), i.e., the optimized value V_(MPPT).

In detail, the DC-DC converter 16 stays off as long as the value of the harvesting signal VIN_(DCDC) is lower than the upper threshold voltage VTH_(UP), as verified in step 45.

As soon as the value of the harvesting signal VIN_(DCDC) exceeds the upper threshold voltage VTH_(UP), step 46 (subsequent to step 45), the enabling signal EN_(DCDC) switches (going, for example, to the high state), enabling the DC-DC converter 16, which is consequently turned on.

It should be noted that switching of the same enabling signal EN_(DCDC) further controls closing of the second comparison switch SW2 _(COMP), so that the second input terminal of the comparator 35 goes to the lower threshold voltage VTH_(DOWN), thus guaranteeing hysteretic operation of the comparator 25.

Furthermore, activation of the DC-DC converter 16 entails a decrease in the value of the harvesting signal VIN_(DCDC), since the average current in the DC-DC converter 16 (when it is on) is higher than the current supplied by the transducer 12. In this step, the capacitance of the storage capacitor 15 is thus discharged with substantially constant current (when, instead, the DC-DC converter 16 is off, the same capacitance is charged by the transducer 12).

In the same step 46, count of a refresh time interval is started, following upon which, as described in detail hereinafter, the values of the sampled voltage V_(S&H) and of the threshold voltages VTH_(UP), VTH_(DOWN) will have to be updated.

The DC-DC converter 16 remains on as long as the value of the harvesting signal VIN_(DCDC) is higher than the lower threshold voltage VTH_(DOWN), as verified in step 47.

As soon as the value of the harvesting signal VIN_(DCDC) drops below the lower threshold voltage VTH_(DOWN), the enabling signal EN_(DCDC) switches again (going, for example, to the low state), disabling the DC-DC converter 16, which is consequently turned off (step 48).

Switching of the same enabling signal EN_(DCDC) further controls opening of the second comparison switch SW2 _(COMP) so that the second input terminal of the comparator 35 once again goes to the upper threshold voltage VTH_(UP).

Furthermore, deactivation of the DC-DC converter 16, entails an increase in the voltage of the harvesting signal VIN_(DCDC) on account of the current drawn by the transducer 12.

The aforesaid steps of increase and decrease of the value of the harvesting signal VIN_(DCDC) (and consequently of the transduction signal V_(TRANSD)) repeats one after the other until the refresh time interval reaches a desired value (this value may conveniently be regulated, also during operation of the circuit), as verified in the same step 48.

In this case, from step 48, control returns to the initial step 40, for a new sampling of the open-circuit voltage V_(OC) supplied by the transducer 12, and refresh of the sampled voltage values and of the threshold values, in a manner altogether similar to what has been described previously.

The operation described will be better understood with reference also to the diagrams of FIG. 5, which show the plots of the harvesting signal VIN_(DCDC) and of the transduction signal V_(TRANSD), during the steps of tracking of the MPPT condition, with duration T_(MPPT), and of refresh and sampling, with duration T_(SAMPLE); further indicated are the time intervals in which the DC-DC converter 16 is on and those in which the DC-DC converter 16 is off, in each on/off cycle, having a duration T_(CYCLE).

In particular, the time interval T_(MPPT) is much longer than the time interval T_(SAMPLE): T_(MPPT)>>T_(SAMPLE).

In general, time interval T_(MPPT) depends on the field of application, in particular on the fact that the environmental conditions to which the transducer 12 is subjected change rapidly; for example, the time interval T_(SAMPLE) may be of the order of some tens of milliseconds (for example, 25 ms) and the time interval T_(MPPT) may range from some seconds to some tens of seconds.

The above characteristic advantageously enables a considerable reduction of the average current consumption. During the tracking step, in effect, only the comparator 25 is on, with a resulting extremely low current consumption.

Purely by way of example, in the aforesaid FIG. 5, the open-circuit voltage V_(OC) supplied by the transducer 12 increases with each refresh of the values sampled. It is, however, altogether evident that, upon start of the refresh time interval, the value of the transduction signal V_(TRANSD) may vary arbitrarily (for example, as a function of the changed environmental conditions, in terms of temperature or conditions of lighting), thus being able to increase, or even decrease, with respect to the immediately preceding step, according to the environmental stimulus.

As illustrated in FIGS. 6 a, 6 b, corresponding to an increase of the open-circuit voltage V_(OC) supplied by the transducer 12 is a decrease in the time interval T_(CYCLE), defined previously; thus, a larger number of cycles during the step of tracking of the MPPT condition occur.

By way of example, FIG. 6 a refers to a voltage V_(OC) of 0.4 V, whereas FIG. 6 b refers to a voltage V_(OC) of 5 V.

FIG. 7 shows a possible circuit implementation for providing the first and second voltage-generator modules 32, 34, which enables substantially negligible leakage currents to be obtained.

In particular, the harvesting interface 14 (of which elements already described previously with reference to FIG. 3 are not described again) comprises in this case a voltage-tracking amplifier 50, having its input connected to the third internal node N₃ and its output defining a sixth internal node N₆, onto which the sampled voltage V_(S&H) is thus brought.

Connected between the sixth internal node N₆ and the reference terminal GND is a first mirroring resistor 51 with resistance R_(S), through which a mirroring current I_(S), equal to V_(S&H)/R_(S), is consequently generated.

The harvesting interface 14 further comprises a current mirror 52 (obtained in a per se known manner, not described in detail herein), having a mirroring branch connected to the sixth internal node N₆ and a mirrored branch connected to a seventh internal node N₇, on which the mirroring current I_(S) is mirrored.

Connected between the seventh internal node N₇ and the reference terminal GND is a second mirroring resistor 54 with the same resistance R_(S), so that on the seventh internal node N₇ there is the same sampled voltage V_(S&H).

The first voltage-generator module 32 is formed in this case by: a first current generator 55, which may be selectively connected to the seventh internal node N₇ by a second decoupling switch SW2 and generates a reference current I_(ref); and a third decoupling switch SW3, designed to connect the seventh internal node N₇ selectively to the fourth internal node N₄, on which the upper threshold voltage VTH_(UP) is present during operation. The first current generator 55 may be obtained in any known way.

The second voltage-generator module 34 is in turn formed by: a second current generator 57, which may be selectively connected to the same seventh internal node N₇, as an alternative to the first current generator 55, via a fourth decoupling switch SW4, and generates the same reference current I_(ref); and a fifth decoupling switch SW5, designed to connect the seventh internal node N₇ selectively to the fifth internal node N₅, on which the lower threshold voltage VTH_(DOWN) is present during operation. Also the second current generator 55 may be obtained in any known way.

The second, third, fourth, and fifth decoupling switches receive respective control signals from the same timing stage 25 (in a way not illustrated here, and as it has been discussed previously).

Operation of the circuit described envisages, as mentioned previously, in an initial step, sampling of the open-circuit voltage of the transducer 12 by closing of the sampling switch SW_(S&H) and of the first decoupling switch SW1, and consequent generation on the third internal node N3 of the sampled voltage V_(S&H), having a value corresponding to the optimized value V_(MPPT).

Next, the sampling switch SW_(S&H) and the first decoupling switch SW1 are both opened, and the sampled voltage V_(S&H) is held on the first holding capacitor C1 _(S&H).

Next, but once again within the sampling time interval T_(SAMPLE), the second and third decoupling switches SW2, SW3 are first closed (with the fourth and fifth decoupling switches SW4, SW5 open) so that on the seventh internal node N7, and consequently on the fourth internal node N4, the voltage VTH_(UP)=V_(S&H)+I_(REF)·R_(S) is generated (note that the aforesaid offset voltage V_(OS) consequently corresponds here to I_(REF)·R_(S)).

Next, once again within the sampling time interval T_(SAMPLE), the second and third decoupling switches SW2, SW3 are opened, and the fourth and fifth decoupling switches SW4, SW5 are closed so that the voltage VTH_(DOWN)=V_(S&H)+I_(REF)·R_(S) is generated on the seventh internal node N7, and consequently on the fifth internal node N5.

Next, also the fourth and fifth decoupling switches SW4, SW5 are opened so that the upper and lower threshold voltages VTH_(UP), VTH_(DOWN) are held on the respective second and third holding capacitors C2 _(S&H), C3 _(S&H) (for the entire duration of the subsequent tracking time interval T_(MPPT)), with minimal leakage currents through the open switches.

In this regard, the present Applicant has realized: a substantially maximum dispersion by the holding capacitors C2 _(S&H), C3 _(S&H) equal to 10 mV/s with an open-circuit voltage V_(OC) of 0.4 V (that is, equal to 5% of the optimal voltage V_(MPPT), in the example 0.2 V, considering a duration of the time interval T_(MPPT) of one second); and a substantially maximum dispersion by the same holding capacitors C2 _(S&H), C3 _(S&H) equal to 100 mV/s with an open-circuit voltage V_(OC) of 5V (equal approximately to 2% of the optimal voltage V_(MPPT), in the example 2.5V, considering once again a duration of the time interval T_(MPPT) of one second).

The performance achieved by the harvesting interface 14 is further highlighted by the plots of FIGS. 8 a-8 d, which refer to use with a transducer 12 of a thermoelectric type, having the following characteristics: series resistance R_(TEG) of 20 kΩ, open-circuit voltage V_(OC) comprised between 1 V (minimum value) and 5 V (maximum value).

In detail, FIG. 8 a shows the average consumption of the S&H stage 22 as the average power delivered by the transducer 12 (available power P_(MAX)) varies. It should be noted that this consumption is very contained and barely dependent upon the average power delivered, and thus barely dependent upon the environmental conditions in which the transducer 12 operates and upon the electrical characteristics of the same transducer 12.

FIG. 8 b shows the plot of the factor η_(Loss) (see the foregoing discussion) as a function of the same power P_(MAX).

FIG. 8 c shows the plot of the factor η_(COUPLE) (see the foregoing discussion), once again as a function of the power P_(MAX).

FIG. 8 d shows the plot of the efficiency η_(MPPT)(see the foregoing discussion) once again as a function of the power P_(MAX).

In particular, it is to be noted that the efficiency η_(MPPT) is higher than 90% with an available power of 20 μW, and rises above 98% with an available power higher than 100 μW.

The advantages of the proposed solution emerge clearly from the foregoing description.

In particular, the hysteretic voltage control for tracking of the MPPT condition enables a considerable saving in power consumption, for example as compared to a continuous-time control solution.

For instance, the condition T_(MPPT)>>T_(SAMPLE) enables considerable accuracy in tracking of the aforesaid MPPT condition and a very high value of the factor η_(COUPLE) to be achieved.

Furthermore, the harvesting interface 14 does not envisage the use of further DC/DC converters to ensure the MPPT condition, thus reducing the number of external components required; a voltage control is in fact implemented, instead of a duty-cycle control, as in many known solutions.

In this regard, it is further pointed out that in the solution described, the holding capacitors are advantageously all obtained using integrated technology, not as external components (in fact, thanks to the reduction of the leakage currents, the value of the same capacitors may not be high).

The solution described further proves very flexible, enabling easy adaptation to use with different photovoltaic or thermoelectric cells; in particular, it is sufficient to regulate the division factor of the sampled voltage V_(S&H) via the voltage divider 30 to obtain a condition of improved matching.

Furthermore, once the operating mode has been selected, with a thermoelectric cell or a photovoltaic cell, the efficiency of tracking of the MPPT condition is found to be independent of the electrical parameters of the transducer 12, for example of the corresponding equivalent resistance or the corresponding open-circuit voltage.

As mentioned previously, the energy harvesting system may advantageously be used for electrical supply of a device, which may even be without any battery, or equipped with a rechargeable battery.

By way of example, FIG. 9 shows an electronic bracelet or watch 60, which incorporates the energy harvesting system for generation of electrical energy by exploiting the difference of temperature between the environment and the human body (the transducer 12 is in this case of a thermoelectric type). The electronic bracelet 60 may advantageously be used in the field of fitness, for example for counting the steps made by a user.

Finally, it is clear that modifications and variations may be made to what has been described and illustrated herein, without thereby departing from the scope of the present disclosure, as defined in the annexed claims.

In particular, the energy harvesting system 10 may comprise a plurality of transducers 12, all of the same type or of a type different from one another.

Furthermore, it is evident that the energy harvesting system 10 may advantageously be used for other applications and other electronic devices, for example in the automotive field, or also in a mobile electronic device or in a garment or other article of clothing, for example in footwear, in the consumer electronics field (any mobile application), the industrial field (for example, in controlling processes that involve environments with high thermal gradients), or in the field of home automation (for example, in combination with photovoltaic generators).

The various embodiments described above can be combined to provide further embodiments. These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure. 

1. An electrical-energy harvesting system, comprising: a transducer to convert energy from an environmental energy source into a transduced signal; a harvesting interface to receive the transduced signal on an input terminal and to supply an electrical-energy harvesting signal on an output terminal; an energy storage element coupled to the output terminal of the harvesting interface to receive the electrical-energy harvesting signal; and a voltage converter coupled to the output terminal to generate a regulated voltage, wherein the harvesting interface is configured to: sample an open-circuit voltage value of the transduced signal; generate an optimized voltage value starting from the open-circuit voltage value; generate an upper threshold voltage based on the optimized voltage value; generate a lower threshold voltage based on the optimized voltage value; and control the voltage converter in a switching mode to thereby maintain the electrical-energy harvesting signal between the upper and lower threshold voltages in a first operating condition.
 2. The system according to claim 1, wherein the harvesting interface is configured to: couple the output terminal to the input terminal during said first operating condition in a condition of maximum power-point tracking; and decouple the output terminal from the input terminal during a second operating condition of sampling said open-circuit voltage value; and generate said upper and lower threshold voltages by said harvesting interface during the second operating condition.
 3. The system according to claim 2, wherein the harvesting interface comprises: a tracking switch coupled between the input terminal and the output terminal to select said first operating condition or said second operating condition.
 4. The system according to claim 3, wherein the harvesting interface comprises: a sample-and-hold stage configured, during said second operating condition, to sample the open-circuit voltage value of the transduced signal, with the tracking switch in the open state, and configured to generate the optimized voltage value, the upper threshold voltage, and the lower threshold voltage; a comparator stage with a hysteretic voltage control based on the upper and lower threshold voltages, the comparator stage configured to generate an enabling signal for the voltage converter to maintain the electrical-energy harvesting signal and, with the tracking switch in the closed state, the transduced signal, between the upper and lower threshold voltages in said first operating condition.
 5. The system according to claim 2, wherein the harvesting interface further comprises: a timing stage to generate control and timing signals to first determine said first operating condition and then to determine said second operating condition.
 6. The system according to claim 5, wherein said timing stage is configured to: cyclically determine execution of said second operating condition to direct updating said open-circuit voltage value and said upper and lower threshold voltages and execution of said first operating condition following upon each new execution of said second operating condition.
 7. The system according to claim 1, wherein said optimized voltage value corresponds to a condition of substantially maximum coupling between said transducer and said harvesting interface.
 8. The system according to claim 1, wherein: VTH_(DOWN)<V_(MPPT)<VTH_(UP), where V_(MPPT) is the optimized voltage value, VTH_(DOWN) is the lower threshold voltage, and VTH_(UP) is the upper threshold voltage.
 9. The system according to claim 1, wherein the harvesting interface comprises a voltage divider to generate the optimized voltage value starting from a sampled value of said open-circuit voltage, wherein a division factor of said voltage divider is configurable as a function of characteristics of said transducer to correspond said optimized voltage value to a condition of substantially maximum coupling between said transducer and said harvesting interface.
 10. The system according to claim 9, wherein the division factor is substantially equal to 0.5 when the transducer is of a thermoelectric type and wherein the division factor is between about 0.75 and about 0.9 in the case where the transducer is of a photovoltaic type.
 11. The system according to claim 1, wherein the harvesting interface comprises: a first holding element to store said optimized voltage value; a second holding element to store said upper threshold voltage; a third holding element to store said lower threshold voltage during said first operating condition; and controllable decoupling-switching elements to selectively decouple said first, second, and third holding elements during a storage condition of each respective voltage value.
 12. The system according to claim 1, wherein the harvesting interface comprises: a first voltage-generator module to generate said upper threshold voltage starting from said optimized voltage value; and a second voltage-generator module to generate said lower threshold voltage starting from said optimized voltage value, wherein said upper threshold voltage and said lower threshold voltage differ from said optimized voltage value by a same offset value.
 13. An electronic device, comprising: a transducer to convert energy from an environmental or mechanical energy source into a transduced signal; a harvesting interface to receive the transduced signal on an input terminal and to supply an electrical-energy harvesting signal on an output terminal; an energy storage element coupled to the output terminal of the harvesting interface to receive the electrical-energy harvesting signal; a voltage converter coupled to the output terminal to generate a regulated voltage, wherein the harvesting interface is configured to: control the voltage converter in a switching mode to thereby maintain the electrical-energy harvesting signal between an upper threshold voltage and a lower threshold voltages in a first operating condition; generate said upper and lower threshold voltages during a second operating condition; and cyclically execute said first operating condition following upon each new execution of said second operating condition; and an electrical load to receive the regulated voltage.
 14. The electronic device according to claim 13, wherein said electrical load includes a rechargeable battery.
 15. The electronic device according to claim 13, wherein said transducer includes at least one of a photovoltaic and a thermoelectric device.
 16. The electronic device according to claim 13, wherein said electronic device is a mobile electronic device, an automotive device, an industrial device, or an article of clothing.
 17. An electrical-energy harvesting method, comprising: by a transducer, converting energy from an environmental energy source into a transduced signal; by a harvesting interface: receiving the transduced signal on an input terminal; supplying an electrical-energy harvesting signal on an output terminal; sampling an open-circuit voltage value of the transduced signal; generating an optimized voltage value starting from the open-circuit voltage value; generating an upper threshold voltage based on the optimized voltage value; generating a lower threshold voltage based on the optimized voltage value; and controlling a voltage converter in a switching mode to maintain the electrical-energy harvesting signal between the upper and lower threshold voltages in a first operating condition; by an energy storage element coupled to the output terminal of the harvesting interface, receiving the electrical-energy harvesting signal and storing electrical energy; and by the voltage converter coupled to the output terminal, generating a regulated voltage.
 18. The method according to claim 17, further comprising, by the harvesting interface: coupling the output terminal to the input terminal during said first operating condition in a condition of maximum power-point tracking; decoupling the output terminal from the input terminal during a second operating condition of sampling said open-circuit voltage value; and generating said upper and lower threshold voltages during the second operating condition.
 19. The method according to claim 18, further comprising, by a timing stage of the harvesting interface: generating control and timing signals to first determine said first operating condition and then to determine said second operating condition.
 20. The method according to claim 19, further comprising, by the timing stage: cyclically determining execution of said second operating condition; updating said open-circuit voltage value and said upper and lower threshold voltages; and executing said first operating condition following upon each new execution of said second operating condition. 